Need help rewriting userid

I have a machine where I have a unix account jblow (for example),
say, jblow@machine.example.com.

However, when sending email, I want this to be rewritten as
joe@example.com.

Rewriting the domain part I can do, via MASQUERADE_AS, but I am not
sure how to tell sendmail to "use joe instead of jblow in outgoing
emails".

Any ideas?

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